Physical entities sometimes possess characteristics which vary randomly from entity to entity. This variability can provide for a Physically Unclonable Function (PUF) which can be used in security technology. Security circuits use this PUF ‘fingerprint’ as an input to generating further random codes related with across-network communications or for gating access to secure parts of an electronic device or system. PUF circuits can, for example, generate a random bit word of 256 to 1024 bits (or more) which can be used to generate a recognized response from an input challenge, to authenticate the user and hardware. The random bit pattern generated by the PUF can represent a unique ‘fingerprint’ or serial number for each specific occurrence of a functional electronic die. The randomness of the PUF generated word is dependent upon the randomness of the varying physical quantity, and specifically how the physical randomness is captured by the PUF circuit.